What is flouride, and why is it good for my teeth?
Flouride is a compound of the element flourine, which is found universally throughout nature in water, soil, air and in most foods. Existing abundantly in living tissue as an ion, flouride is absorbed easily into tooth enamel, especially in children's growing teeth. Once teeth are developed, fluoride makes the entire tooth structure more resistant to decay and promotes remineralization, which aids in repairing early decay before the damage is even visible.
"Systemic" fluoride is ingested when added to public and private water supplies, soft drinks and teas, and is available in dietary supplement form. Once systemic fluoride is absorbed via the gastro-intestinal tract, the blood supply distributes it throughout the entire body. Most fluoride not excreted is deposited in bones and hard tissues like teeth.
Excerpt from "Flouride & Your Health" Fact Sheet provided by the Academy of General Dentistry